In general, a hollow fiber membrane module is classified broadly into an internal pressure type and an external pressure type. The external pressure type of hollow fiber membrane module normally has a structure of bundling several hundreds to tens of thousands of hollow fiber membranes with a length of 200 to 3,000 mm and an outside diameter of the membrane of 0.1 to 5 mm, accommodating the bundle in a cylindrical case, and adhesively fixing the ends of both sides to an inner wall of the case with a resin. When adhesively fixing both ends, there are an one-end collection type module and a both-ends collection type module: the former is formed so as to have the end of the hollow fiber membrane opened in one adhesively-fixed part, have the hollow part of fiber membrane sealed at the other adhesively-fixed part, supply compressively raw water to a region sandwiched between the adhesively-fixed parts to permeate through the hollow fiber membrane, and take the filtrate out from the adhesively-fixed part in which the end of the hollow fiber is opened; and the latter is formed so as to have the ends of the hollow fiber membrane opened in both adhesively-fixed parts, and take the filtrate out from both ends. Furthermore, there are a plurality of through-holes in hollow fiber membrane bundles at the adhesively-fixed part that becomes to be a lower side when being used, and their ports are used as a port for supplying raw water to be filtrated, and as an air supply port and a cleaning waste water outlet in a physical cleaning process.
When such an external pressure type hollow fiber membrane module is used for the purpose of bacteria eliminating and clarification, it is normally subjected to a cross flow filtration to prevent a suspended solid from depositing on the surface of a hollow fiber membrane, or to the periodic physical cleaning such as a back wash reverse filtration and an air bubbling to recover a filter performance, thereby enabling a stable filtration operation. In order to operate the module in the above method, the module has an exhaust port provided at the side face of a case in the vicinity of an adhesively-fixed part in an upper part, and is vertically installed. When the module is used for filtration, the raw water containing the suspended solid is supplied from a through-hole provided in the adhesively-fixed part in the lower part, and the concentrated water is discharged from the exhaust port provided in the side face in the upper part of the case. In addition, in a cleaning step by air bubbling, the module is cleaned by the steps of supplying air from the through-hole in the lower part; thereby forming an air/water mixture flow; making the membrane oscillated by the air/water mixture flow; thereby stripping off the suspended solid deposited on the surface of a membrane; then supplying raw water together with air; and discharging it from the exhaust port provided at the side face in the upper part of the case.
The flow of a liquid discharged from the exhaust port in such a cross flow filtration and an air bubbling cleaning occasionally draws the hollow fiber membrane into the exhaust port to damage it, and the oscillation of the membrane occasionally makes stress concentrated in the vicinity of the inner surface of an adhesively-fixed part to rupture the hollow fiber membrane.
In order to efficiently discharge a deposit outside a module by exfoliating it from the surface of a membrane by the physical cleaning as described above, a space must be secured between hollow fibers, so that the hollow fiber membrane cannot be accommodated in the module having approximately the closest packed state, as in the case of an internal pressure type module. For this reason, a membrane-occupying rate in a housing is normally set at 0.3 to 0.6. The “membrane-occupying rate in a housing” used herein means a ratio of a total of the cross-sectional areas based on the outside diameter of the membrane with reference to the cross-sectional area based on the inside diameter of the housing in a filtration region of the membrane. When the membrane is accommodated in a comparative low density, the distribution of the membrane in an adhesively-fixed part of the membrane easily tends to be ununiform, and the hollow fiber membrane in the vicinity of the inner surface of the above adhesively-fixed part develops a strong tendency to be ruptured.
As the means of preventing the above damage/rupture of a membrane in the vicinity of the adhesively-fixed part of the hollow fiber membrane, a method has been known which installs a current cylinder in the vicinity of the inner surface of the adhesively-fixed part and arranges a layer of a high polymer material having rubber-like elasticity in the inner side of the adhesively-fixed part (for instance, see Patent Document 1). A method is also proposed which coats the surface of the hollow fiber membrane extending in the inner side of the adhesively-fixed part with the same adhesive as that for forming the inner surface of the fixed part (for instance, see Patent Document 2).
However, the method according to Patent Document 1 has an excellent effect in preventing the damage/rupture of a membrane as described above, but when forming an adhesively-fixed layer only with a high polymer having rubber-like elasticity, this may cause the problem that the adhesively-fixed part may be ruptured because the pressure resistance of the part is inferior. In addition, in order to secure the pressure resistance, it is required to adhesively fix the membrane with a material having high strength and high elasticity, and then to form a layer of a rubber-like elastic body on the inner side thereof, so that the method has a disadvantage of needing a complicated manufacturing process and a high cost.
On the other hand, the method according to Patent Document 2 does not have a sufficient effect of preventing the membrane from being ruptured, and causes the problem that the membrane is ruptured by air bubbling in a long term operation.    Patent Document 1: JP-A-09-220446    Patent Document 2: JP-A-10-305218